Field of the Invention
This invention relates to a hydrostatic drive system with a hydrostatic pump which is driven by a drive motor and is connected in a closed circuit with a hydrostatic motor. The hydrostatic motor is in a drive connection with a consumer. The closed circuit is formed by a first hydraulic connection and a second hydraulic connection. A hydrostatic accumulator device can be connected with at least one of the two hydraulic connections for the storage of energy and the discharge of energy. A hydrostatic drive unit is located in an accumulator flow path between one of the two hydraulic connections and the hydrostatic accumulator and an additional hydrostatic drive unit is located in a tank flow path between the other of the two hydraulic connections and a tank.
Description of Related Art
Hydrostatic drive systems in a closed circuit are used for driving a consumer in vehicles such as motor vehicles or public transit vehicles, e.g. buses, preferably for urban mass transit, and for mobile self-propelled machines, in particular industrial trucks, agricultural machines, forestry machines and construction machines, such as, excavators, wheel, and telescoping loaders, tractors, combine harvesters, forage harvesters, sugar beet and potato harvesters.
With a hydrostatic accumulator for the storage of energy and the discharge of energy that can be connected with at least one of the two hydraulic connections of the closed circuit, a hybrid drive concept, and thus a hybrid function, can be realized in a hydrostatic drive system with a closed circuit. The consumer can be driven in a drive phase with the energy in the hydrostatic accumulator during a discharge operation of the hydrostatic accumulator. If the hydrostatic accumulator is operated in a charging operation during a braking or deceleration phase of the consumer and is charged with hydraulic fluid, energy can be recovered during the braking or deceleration of the consumer.
In a hydrostatic drive system in a closed circuit, it should be ensured that during the charging operation of the hydrostatic accumulator, during which a determined amount of hydraulic fluid is removed from the hydrostatic accumulator out of the high-pressure side hydraulic connection of the closed circuit, the corresponding quantity of hydraulic fluid is made up at the low-pressure side hydraulic connection of the closed circuit. Correspondingly, during discharge operation of the hydrostatic accumulator, during which a determined quantity of hydraulic fluid is fed from the hydrostatic accumulator to the high-pressure side hydraulic connection of the closed circuit, it should be ensured that the corresponding quantity of hydraulic fluid is removed from the low-pressure side hydraulic connection of the closed circuit.
To make possible this quantity and/or volumetric balancing of the closed circuit at the low-pressure side hydraulic connection during the extraction or the addition of a quantity of hydraulic fluid at the high-pressure side hydraulic connection by the hydrostatic accumulator, it is known that two additional hydrostatic drive units can be provided that balance the quantity and/or the volume of hydrostatic fluid at the two hydraulic connections of the closed circuit.
A hydrostatic drive system with a hydrostatic accumulator and two additional hydrostatic drive units that balance the quantity and/or volume at the two hydraulic connections of the closed circuit during the charging and discharging of the hydrostatic accumulator is described in DE 10 2009 011 247 A1. A first hydrostatic drive unit is located in an accumulator flow path between one of the two hydraulic connections and the hydrostatic accumulator, and is driven by the hydraulic fluid fed from the high-pressure side of the closed circuit to the hydrostatic accumulator, or by the hydraulic fluid diverted from the hydrostatic accumulator into the high-pressure side of the closed circuit. This first hydrostatic drive unit is in a drive connection with a second hydrostatic drive unit which is located in a tank flow path between the other of the two hydraulic connections and a tank. The hydraulic fluid delivered to the hydrostatic accumulator from the high-pressure side of the closed circuit or the hydraulic fluid diverted from the hydrostatic accumulator into the high-pressure side of the closed circuit therefore drives the first hydrostatic drive unit and the second hydrostatic drive unit (which is in a drive connection with the first hydrostatic unit). The second hydrostatic drive unit delivers an equivalent quantity of hydraulic fluid into the low-pressure side of the closed circuit and diverts the corresponding quantity of hydraulic fluid from the low-pressure side of the closed circuit to the tank. The first drive unit and the second drive unit can have the same displacement volume and thus an equal volumetric flow of hydraulic fluid, can be coupled with each other by means of the shaft, and can be in a drive connection. Alternatively, it is possible for the first drive unit and the second drive unit to have different displacement volumes, wherein the two drive units are coupled with each other and in a drive connection by means of the transmission, such as a spur gear transmission. The transmission translation ratio of the transmission is designed so that the two drive units displace an equal volumetric flow of hydraulic fluid. Because the two drive units displace the same volumetric flow of hydraulic fluid, the second hydrostatic drive unit (which is located between the low-pressure side hydraulic connection of the closed circuit and the tank) delivers or diverts exactly the quantity of hydraulic fluid to the low-pressure side hydraulic connection of the closed circuit as is diverted at the high-pressure side hydraulic connection into the hydrostatic accumulator (or is delivered from the hydrostatic accumulator into the high-pressure side hydraulic connection) so that the quantity and volume of hydraulic fluid in the two hydraulic connections of the closed circuit is balanced.
In DE 10 2009 011 247 A1, the two additional hydrostatic drive units that balance the quantity and/or volume of hydraulic fluid are each in the form of individual drive units which are connected with each other and are in a drive connection by means of a shaft or transmission. This design entails a great deal of construction effort and expense and occupies a large amount of space, because each of the two individual drive units has a separate housing and a separate shaft with the corresponding shaft bearing in the respective housing of the corresponding individual drive unit. The shaft required for the coupling of the two individual drive units or the transmission required for the coupling of the two individual drive units also entails a great deal of construction effort and expense and occupies a large amount of space. The hybrid function described in DE 10 2009 011 247 A1 with a hydrostatic accumulator and two additional hydrostatic drive units, each of which is in the form of an individual drive unit, therefore occupies a large amount of space, entails a great deal of extra manufacturing effort and expense, and makes the entire system more complex and expensive.
Therefore, it is an object of the invention to provide a hydrostatic drive system in which the balancing of the quantity and/or volume of hydraulic fluid with the hybrid function by the hydrostatic accumulator has a simple, compact, and/or economical construction.